Preheat your oven to 180c and line a cupcake tray with cases.
Combine the milk and apple cider vinegar and set aside to allow this to thicken to a buttermilk consistency.
Meanwhile, mix together all of your dry ingredients. Then fold through the olive oil, milk mixture and mashed banana.
Once nice and smooth, divide your batter between the cupcake cases and bake these for 30 minutes, or until cooked through. Allow to cool completely.
Whilst your cakes are cooling, beat the butter until light and fluffy (I use and electric whisk for this). Gradually sift in the icing sugar and cocoa powder whilst continuing to whisk. Add in your coffee and whisk a little more! You should be left with a luciously thick and creamy frosting.
Pipe this onto your cakes and garnish with your favourite toppings!